Cutting Rotor for Granulating Plastic Castings

ABSTRACT

The invention relates to a rotary cutter for the pelletization of plastic extrudates, said rotary cutter being provided with knives projecting axially away from its end surface, said knives being individually affixed to supporting surfaces of the rotary cutter by means of fastening elements. The supporting surfaces are formed by the side walls of radial grooves in the peripheral surface of the rotary cutter and by radial transverse walls in the radial grooves, said radial grooves being limited by the radial transverse walls, the knives each being inserted in hook-like manner with a projecting portion into said radial transverse walls, wherein one side of the projecting portion forms an abutment for the fastening element, said fastening element penetrating a radial transverse wall and pressing on the abutment.

The invention relates to a rotary cutter for the pelletization ofplastic extrudates, said rotary cutter being provided with knivesprojecting axially away from its end surface, said knives beingindividually affixed to supporting surfaces of the rotary cutter bymeans of fastening elements.

Such a rotary cutter is described and presented in U.S. Pat. No.3,196,487. Said rotary cutter is used in an underwater pelletizer havinga die plate with circularly disposed extrusion orifices, said extrusionorifices being skimmed over in circular motion by the knives of therotary cutter, said knives widely projecting in the manner of blades.The rotary cutter rotates about a shaft extending through the centre ofthe circle along which are disposed the extrusion orifices in the dieplate. As they move over the extrusion orifices, the knives cut theextruded plastic strands issuing in molten form from the extrusionorifices into individual pieces which form the desired pellets orgranules. This cutting operation takes place under water, said waterflowing over the die plate during rotation of the rotary cutter. Owingto their considerable length, the knives, which project in the manner ofblades, tend to flutter somewhat, this having an adverse effect on theclose contact between knives and die plate. However, constant closecontact between the knives and the die plate is a prerequisite for theuniform cutting of the plastic strands.

In another known underwater pelletizer according to DE-PS 10062113, theknives supported by the rotary cutter are individually screwed ontosupporting surfaces affixed to the rotary cutter and provide a morecompact, flutter-free design. The supporting surfaces extend towards thedie plate in the rotary cutter, for which purpose the rotary cutter isprovided with supporting surfaces extending in identical direction. Theindividual knives are pressed onto said supporting surfaces by means ofscrews which serve as fastening elements, said screws penetrating theknives, with the heads of the screws protruding from the knives such asto be easily accessible for a tool. Provided between the individualknives and the individual supports, forming the supporting surfaces, onthe rotary cutter are intermediate spaces through which the water andpellets are able to flow away in the axial direction. It has emergedthat the screw heads, which protrude from the individual knives, are,firstly, not easily accessible on account of the oppositely positionedsupports and, secondly, obstruct the passage of water and pellets as thelatter flow past them.

The object of the invention is to improve the fastening of the knives tosuch a rotary cutter such that, firstly, there is good accessibility ofthe fastening elements which support the knives and that, secondly, saidfastening elements do not obstruct the flow in the aforementionedintermediate spaces.

The object of the invention is achieved in that the supporting surfacesare formed by the side walls of radial grooves in the peripheral surfaceof the rotary cutter and by radial transverse walls in the radialgrooves, said radial grooves being limited by the radial transversewalls, the knives each being inserted in hook-like manner with aprojecting portion into said radial transverse walls, wherein one sideof the projecting portion forms an abutment for the fastening element,said fastening element penetrating a radial transverse wall and pressingon the abutment.

On account of this hook-like manner of fastening of the individualknives and the insertion of the fastening elements through a radialtransverse wall, said fastening elements, as seen from one end surfaceof the rotary cutter, are directly and easily accessible. They aredisposed at a distance from the intermediate spaces between knives andsupports and are therefore unable to obstruct the flow in saidintermediate spaces. Furthermore, the hook-like design of the knivesprovides a high degree of security with regard to their position inrelation to the rotary cutter, with the overall consequence that therotary cutter according to the invention is of a flutter-free designwhich is particularly stable for operation.

In order to provide secure support for the knife, the side of theprojecting portion facing the knife edge may extend at such an angle tothe radial direction that the fastening element, pressing on theprojecting portion and being in the form of a screw, exerts a forcecomponent such as to pull the projecting portion into the rotary cutter.However, it is also possible for the projecting portion to be providedwith a notch, said notch being engaged by a correspondingly shaped endsurface of the fastening element, said fastening element being in theform of a screw. Screws are preferably used as the fastening elements.In this case, it is advantageous for the screw head to be provided witha recess in the rotary cutter, the screw head being countersunk in therotary cutter. The screw head is provided in conventional manner with arecess for receiving a turning tool. The fact that the screw head isaccommodated in a recess in the rotary cutter has, above all, theadvantage that the screw heads are unable to obstruct any cleaningoperations. Preferably, the screws are disposed such that they penetrateinto the rotary cutter from the side facing the knife edge. Said side isparticularly well accessible for repair work. However, it is alsopossible for the screws to be provided in the opposite direction, i.e.such that they penetrate into the rotary cutter from the side facingaway from the knife edge.

Illustrative embodiments are presented in the drawings, in which:

FIG. 1 shows the rotary cutter in a view onto its end surface withscrews as fastening elements;

FIG. 2 shows the same rotary cutter in a view onto the rear side of theknives;

FIG. 3 shows a portion of the rotary cutter with some of the radialgrooves unoccupied;

FIG. 4 shows a section through the rotary cutter in the region of aninserted knife;

FIG. 5 shows a variation on the design of the rotary cutter from FIG. 4,with a notched projecting portion;

FIG. 6 shows a further variation with double knife;

FIG. 7 shows a further variation with a screw from the rear side;

FIGS. 8 and 9 show a further variation with a wedge as fasteningelement.

The rotary cutter 1 presented in FIG. 1 is provided on its peripheralsurface 2 with the individual knives 4, the cutting edges 5 of whichextend radially with respect to the axis of the rotary cutter 1 and thuscontact a die plate (not shown), as presented, for example, in U.S. Pat.No. 3,196,487, wherein, as the rotary cutter 1 rotates with the knives4, the extruded plastic strands are then pelletized in theabove-described manner. The knives 4 are exchangeably fastened to therotary cutter 1 by means of the fastening elements in the form of screws12 (see, for example, FIG. 4). As is apparent from FIG. 1, the screws 12are easily accessible from the end surface 3 of the rotary cutter 1,where they are unable to obstruct a flow in the intermediate spacesbetween the individual knives 4.

FIG. 2 shows the same rotary cutter 1 with the knives 4 which projectfrom the peripheral surface 2 of the rotary cutter 1. The fastening ofthe knives 4 to the rotary cutter 1 is presented in FIG. 4 to 7.

The rear side of the rotary cutter 1 presented in FIG. 2 has threereceiving holes 17 which serve to fasten the rotary cutter 1 to a rotaryshaft (not shown).

FIG. 3 presents a view of the rotary cutter looking perpendicularly ontothe shaft thereof, said view showing the design of the radial grooves,some of which are without knives. As can be seen, the rotary cutter 1 isprovided in the region of its peripheral surface 2 with a plurality ofradial grooves, of which the radial grooves 7 have been left unoccupiedfor the purpose of better representation of the design of the rotarycutter 1. Only one radial groove (not visible) is occupied by the knife4. The radial grooves 7 are limited at the sides by the side walls 8 and9 and also by the radial transverse walls 10 and 11. Consequently, theradial grooves 7 are of a length which, except for a remaining gap,corresponds to the length of the projecting portions 6 (see FIG. 4).

FIG. 4 presents a sectional view along the centre of the knives 4 fromFIGS. 1 and 2 and shows how the individual knives 4 are fastened. Saidknives 4 are each provided with a projecting portion 6 which projectsaway in hook-like manner, said projecting portion 6 projecting into aradial groove 7 in the rotary cutter 1 (see FIG. 3). The radial groove 7is of a width which corresponds, with a small tolerance, to thethickness of the knives 4, with the result that an inserted knife 4 issecurely supported laterally on the side walls 8 and 9 of the radialgroove 7. The knife 4 is secured by the screw 12 which is screwed into acorresponding penetration in the rotary cutter 1 and presses onto theabutment 18 on the projecting portion 6, thereby securely fastening theknife 4 to the rotary cutter 1. The side of the projecting portion 6facing the knife edge 5 extends, as is clearly shown in FIG. 4, at suchan angle to the radial direction that the screw, pressing on theprojecting portion 6, subjects the projecting portion 6 to a force whichpulls the projecting portion 6 into the rotary cutter 1, therebyguaranteeing the secure fastening of the projecting portion 6.

FIG. 5 presents a variation on the design shown in FIG. 4 in which theknife 4 is secured to the rotary cutter 1 by means of a notch 14 in theprojecting portion 6, said notch 14 serving as an abutment. For thispurpose, the screw 15 is provided on its end surface with acorresponding conical portion which fits into the notch 14 and, with thescrew 15 tightened, secures the knife 4 to the rotary cutter 1.

As in the design shown in FIG. 4, also in the design shown in FIG. 5 thescrew 15 is provided with a hexagonal recess into which it is possibleto insert an appropriate key or wrench with which to tighten the screw15.

FIG. 6 presents a variation on the design shown in FIG. 5 in which theknife 4 is double-edged. This design has the advantage that, when one ofits cutting edges 5 is worn, the knife 4 can simply be turned round, asa result of which a new cutting edge comes into the region of the rotarycutter 1 which then performs the pelletizing operation.

FIG. 7 shows a design which is essentially the same as that shown inFIG. 4, it merely being the case that the screw 16 presses against theprojecting portion 6 from the side facing away from the knife edge 5,thereby securing the knife 4 to the rotary cutter 1.

It should also be pointed out that, in the methods of fasteningpresented in FIG. 4 to FIG. 7, the head of the screw in each casedisappears into a countersink, said head being adapted to receive aturning tool. This has the advantage that, when it occasionally becomesnecessary to clean off firmly adhering pelletized material from therotary cutter 1, the scraping off of said material does not result indamage to and therefore breaking off of the screw head, which would thensubsequently make it impossible to unscrew the screw.

In the illustrative embodiments shown in FIG. 1 to 7, screws are used asthe fastening elements for the knives. As initially indicated, however,it is also possible to employ alternative fastening elements. FIGS. 7and 8 present an illustrative embodiment in which a wedge piece is usedas the fastening element, FIG. 8 showing a radial section through therotary cutter 1, wherein, in order to simplify the representation, onlyone knife has been inserted into the rotary cutter 1, while FIG. 9 showsa section along line IX-IX from FIG. 8.

As shown in FIGS. 8 and 9, the wedge piece 19 penetrates into the rotarycutter 1, for which purpose the rotary cutter 1 is provided with acorresponding penetration 20 into which the wedge piece snugly fits withits rear cylindrical neck. With its front wedge-shaped portion, thewedge piece 19 presses laterally against the projecting portion 21,which projecting portion 21 is in this case provided on its side facingthe wedge piece 19 with a corresponding bevelled portion 22. On accountof the friction between it and the bevelled portion 22, the wedge piece22 is securely held in its position as presented in FIGS. 8 and 9. Itshould also be pointed out that the use of a wedge piece may be providedfrom both sides of the rotary cutter 1, as was already presented in theabove-described drawings in connection with the use of screws asfastening elements.

1. Rotary cutter (1) for the pelletization of plastic extrudates, saidrotary cutter (1) being provided with knives (4) projecting axially awayfrom its end surface, said knives (4) being individually affixed tosupporting surfaces of the rotary cutter (1) by means of fasteningelements (12), characterized in that the supporting surfaces are formedby the side walls (8, 9) of radial grooves (7) in the peripheral surface(2) of the rotary cutter (1) and by radial transverse walls (10, 11) inthe radial grooves (7), said radial grooves (7) being limited by theradial transverse walls (10, 11), the knives (4) each being inserted inhook-like manner with a projecting portion (6) into said radialtransverse walls (10, 11), wherein one side of the projecting portion(6) forms an abutment (18) for the fastening element (12), saidfastening element (12) penetrating a radial transverse wall (11) andpressing on the abutment (18).
 2. Rotary cutter according to claim 1,characterized in that the side of the projecting portion (6) facing theknife edge (5) extends at such an angle to the radial direction that thefastening element, pressing on the projecting portion (6) and being inthe form of a screw (12), exerts a force component such as to pull theprojecting portion (6) into the rotary cutter (1).
 3. Rotary cutteraccording to claim 1, characterized in that the projecting portion isprovided with a notch (14), said notch (14) being engaged by acorrespondingly shaped end surface of the fastening element (15), saidfastening element (15) being in the form of a screw.
 4. Rotary cutteraccording to claim 2, characterized in that the head of each of thescrews (12, 15, 16), said head being provided with a recess forreceiving a turning tool, is countersunk in a recess in the rotarycutter (1).
 5. Rotary cutter according to claim 1, characterized in thatthe fastening element is a wedge piece for jamming the projectingportion (21) in the radial groove.
 6. Rotary cutter according to claim1, characterized in that the fastening elements (12) penetrate into therotary cutter (1) from the side facing the knife edge (5).
 7. Rotarycutter according to claim 1, characterized in that the fasteningelements (16) penetrate into the rotary cutter (1) from the side facingaway from the knife edge (5).
 8. Rotary cutter according to claim 3,characterized in that the head of each of the screws (12, 15, 16), saidhead being provided with a recess for receiving a turning tool, iscountersunk in a recess in the rotary cutter (1).
 9. Rotary cutteraccording to claim 2, characterized in that the fastening elements (12)penetrate into the rotary cutter (1) from the side facing the knife edge(5).
 10. Rotary cutter according to claim 3, characterized in that thefastening elements (12) penetrate into the rotary cutter (1) from theside facing the knife edge (5).
 11. Rotary cutter according to claim 4,characterized in that the fastening elements (12) penetrate into therotary cutter (1) from the side facing the knife edge (5).
 12. Rotarycutter according to claim 5, characterized in that the fasteningelements (12) penetrate into the rotary cutter (1) from the side facingthe knife edge (5).
 13. Rotary cutter according to claim 2,characterized in that the fastening elements (16) penetrate into therotary cutter (1) from the side facing away from the knife edge (5). 14.Rotary cutter according to claim 3, characterized in that the fasteningelements (16) penetrate into the rotary cutter (1) from the side facingaway from the knife edge (5).
 15. Rotary cutter according to claim 4,characterized in that the fastening elements (16) penetrate into therotary cutter (1) from the side facing away from the knife edge (5). 16.Rotary cutter according to claim 5, characterized in that the fasteningelements (16) penetrate into the rotary cutter (1) from the side facingaway from the knife edge (5).